JP2014070276A - Large-sized cast member made of nickel based alloy, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a large-sized cast member having high strength and stable properties for long time even at high temperature.SOLUTION: A large-sized cast member contains a nickel alloy having a composition containing Cr:20 to 23 wt.%, Mo:8 to 10 wt.%, Nb+Ta:3.15 to 4.15 wt.%, Fe:0 to 5 wt.%, Co:0 to 1 wt.%, Ti:0 to 0.4 wt.%, Al:0 to 0.4 wt.% and having at least one character of (1) a δ phase or a Laves phase is deposited at a dendrite border and a grain boundary, or (2) a MC type, MCtype or MC type carbide is deposited at the grain boundary. The large-sized cast member can be obtained by applying a heat treatment at 700 to 820°C for 8 to 200 hours to a cast member obtained by casting after a solution treatment.

Description

  The present invention relates to a cast member made of a nickel base alloy, and more particularly to a large cast member used at a high temperature such as a high temperature member of a steam turbine.

  In recent years, development of a thermal power plant (A-USC: Advanced-Ultra Super Critical) having a steam temperature of 700 ° C. or higher is being promoted with the aim of increasing the efficiency of a coal-fired power plant. For high-temperature members of conventional steam turbines, iron-based materials such as 9Cr-based or 12Cr-based heat resistant ferritic steel are used. However, heat-resistant ferritic steel is said to have a limit of 650 ° C. in steam temperature as a use environment, and is considered difficult to apply to a 700 ° C. class steam turbine. Therefore, a nickel (Ni) -based alloy has been studied as a high-temperature member for a steam turbine of 700 ° C. class.

  Nickel-based alloys include many alloys that precipitate stable intermetallic compounds at high temperatures by adding elements such as Al and Ti in addition to Cr and applying appropriate heat treatment, and exhibit excellent high-temperature strength characteristics ( Precipitation strengthening). Al and Ti are prone to segregation, but for materials such as rotor shafts, ingots can be made by a melting method using a double melt process such as VIM + ESR or VIM + VAR or a triple melt process such as VIM + ESR + VAR, depending on the optimal alloy design. A homogeneous member can be obtained by fabrication and forging.

  On the other hand, the steam turbine casing, the steam turbine valve member, and the like are manufactured by casting using a large mold because they are large in size and complicated in shape. However, since the shape is complicated, a melting method such as a double (triple) melt process as described above cannot be applied. In addition, it is difficult to control the atmosphere in casting using a large mold, and it is difficult not only to control the active element components but also to become inclusions due to oxidation of active elements during melting, which can be a defect that adversely affects material properties. .

  Therefore, even for the same nickel-based alloy, in the case of a cast member, application of an alloy strengthened by solid solution strengthening instead of precipitation strengthening has been studied. As such a nickel-based alloy, Alloy 625 disclosed in Patent Document 1 and Patent Document 2 is known. A technique for improving the characteristics of Alloy 625 as described in Patent Document 3 is known.

US Pat. No. 3,046,108 US Pat. No. 3,160,500 JP 2010-261104 A

  The inventors of the present invention made a prototype of a thick specimen assuming a thick member such as a turbine casing by using Alloy 625 and found that it had good manufacturability with no macro defects. Although it has high-temperature strength (creep characteristics), it was confirmed that the proof stress was considerably low. The low proof stress means that, for example, a defect caused by tightening of a bolt occurs when installing a casing or the like, which becomes a big problem in practical use.

  In order to improve the yield strength, refinement of crystal grains is one effective means based on Hall Petch's law. However, in the case of a large cast member, the cooling rate during solidification is slow and the crystal grains tend to be rough. In the case of an iron-based material, the structure can be refined by heat treatment (quenching / tempering) and the proof stress can be improved. However, in a nickel-based alloy, the crystal grain size cannot be controlled by heat treatment. Furthermore, when the cooling rate is slow, microsegregation tends to increase, and there is a difference in components at the boundary between the dendrite core and the dendrite, and it is expected that the microscopic material characteristics will be different even within the same crystal grain.

  Since Alloy 625 is a solid solution strengthening material, it is usually used as it is, and is not subjected to aging heat treatment. Alloy 625 is an alloy having excellent heat resistance and corrosion resistance below 500 ° C., and when exposed to temperatures below 700 ° C., the γ ″ phase (intermetallic compound) precipitates to increase the strength. When exposed, the precipitated γ ”phase is transformed into a δ phase or a Laves phase. Since the δ phase and Laves phase are hard and brittle, they are generally considered to be embrittled phases and are not desired to be precipitated. Moreover, the occurrence of such a transformation means that the material characteristics change greatly, and reliability is an issue in application to a steam turbine that requires stable characteristics over a long period of time.

  In Patent Document 3, a turbine bucket cover is thermo-mechanically formed from an alloy 625 (Alloy 625), and the turbine bucket cover is heat-treated at about 538 ° C. to 760 ° C. for about 100 hours or less, particularly at about 677 ° C. for about 50 hours. A method is described, the heat treatment being used to conserve the dislocation structure and to give secondary strength through the formation of γ ″ precipitates, and to nucleate γ ″ while not forming an equilibrium δ phase. It is described that it is performed so that sufficient time is allowed for growth. However, as described above, since the γ ″ phase is a metastable phase, there is a concern about the stability of the characteristics for a long time. Especially in the A-USC plant, the steam temperature exceeds 700 ° C. There is concern about the precipitation of δ phase and Laves phase.

  Accordingly, an object of the present invention is to provide a large-sized cast member made of Alloy 625 that has a high yield strength and has characteristics that are stable for a long time even at high temperatures.

  As a result of examining the problems as described above, the present inventors have been able to improve the yield strength by positively precipitating a δ phase or a Laves phase by heat treatment in a large cast member manufactured using Alloy 625. I found.

The present invention is Cr: 20-23 wt%, Mo: 8-10 wt%, Nb + Ta: 3.15-4.15 wt%, Fe: 0-5 wt%, Co: 0-1 wt%, Ti: It consists of a nickel-base alloy having a composition containing 0 to 0.4% by weight, Al: 0 to 0.4% by weight, and inevitable impurities. (1) A δ phase or Laves phase is precipitated at the dendrite boundaries and grain boundaries. Or (2) a large cast member having at least one of the following characteristics: MC type, M ₂₃ C ₆ type or M ₆ C type carbide is precipitated at the grain boundaries.

  Moreover, this invention is a manufacturing method of the said large sized cast member, Comprising: Cr: 20-23 weight%, Mo: 8-10 weight%, Nb + Ta: 3.15-4.15 weight%, Fe: 0-5 weight %, Co: 0 to 1% by weight, Ti: 0 to 0.4% by weight, Al: 0 to 0.4% by weight, and casting using a nickel-based alloy having a composition containing inevitable impurities, and It is related with the manufacturing method of the large sized cast member including the process of heat-processing the obtained cast member at 700-820 degreeC for 8 to 200 hours after solution treatment.

  According to the present invention, it is possible to provide a large-sized cast member having sufficient proof strength and stable characteristics for a long time even at high temperatures. The large cast member of the present invention is suitable as a member of a steam turbine, particularly as a member of a steam turbine having a steam temperature exceeding 700 ° C.

It is a general TTT (Time-Temperature-Transformation) diagram of Alloy625.

  The nickel-base alloy used in the present invention is composed of 20 to 23% by weight of Cr, 8 to 10% by weight of Mo, 3.15 to 4.15% by weight of Nb and Ta, 0 to 5% by weight of Fe, Co The chemical composition contains 0 to 1 wt% of Ti, 0 to 0.4 wt% of Ti, and 0 to 0.4 wt% of Al, and further contains inevitable impurities. Such a nickel-based alloy is called Alloy 625 and is commercially available. Hereinafter, a nickel-base alloy having such a chemical composition is referred to as Alloy 625 in this specification.

  Normally, Alloy 625 is used as a solution treatment and has excellent corrosion resistance and heat resistance. However, it is known that intermetallic compounds and carbides precipitate in a high temperature environment of 600 ° C. or more, and the characteristics change greatly. Yes. FIG. 1 shows a general TTT (Time-Temperature-Transformation) diagram of Alloy 625.

  In the method described in Patent Document 3, the strength is improved by heat-treating Alloy 625 to precipitate the γ ″ phase. The heat treatment conditions of the method of Patent Document 3 can also be seen from the TTT diagram of FIG. Thus, it is a condition that the δ phase and the Laves phase are difficult to precipitate.In fact, Patent Document 3 states that the δ phase must not be formed, and that the γ ”phase has an operating temperature of less than 649 ° C. It is specified that it is stable and does not return to the undesirable δ phase. However, since the γ ″ phase is a metastable phase, when it is exposed to a high temperature for a long time, it changes to another intermetallic compound such as δ phase or Laves phase. According to the TTT diagram of FIG. If this state continues for a long time, the γ ”phase is precipitated first, but after about 100 hours, the Laves phase and the δ phase start to precipitate. Thermodynamically, the Laves phase and the δ phase are more stable than the γ ″ phase, so the γ ″ phase will eventually disappear. The γ ″ phase is also used in other alloys (such as Alloy 718) as a useful phase for increasing the strength. However, if the phase change from γ ″ to δ phase or Laves phase occurs, the characteristics of the member There are concerns that it will change significantly. This is not preferable for a device (for example, a steam turbine) that operates for a long time (tens of thousands to hundreds of thousands of hours), which may cause a decrease in reliability.

  In particular, in the case of a large cast product, since the solidification rate is slow, the structure (dendrites, crystal grains) increases, and the microsegregation also increases. This means that there is compositional variation among the same crystal grains, and there is a concern that the precipitation behavior does not show the behavior as shown in the TTT diagram of FIG.

  The inventors actually made a large cast member of Alloy 625, and after performing solution treatment, and conducted a test for aging heat treatment under various conditions (time and temperature), dendritic boundary and dendritic core, grain boundaries, It was confirmed that the precipitation behavior differs depending on the location. Moreover, when the hardness was measured as a simple mechanical property evaluation, it was confirmed that the hardness greatly varied depending on the location. This is considered to be due to the fact that precipitates are likely to precipitate because the elements such as Nb are concentrated at the dendritic boundary, and conversely, because the dendritic core does not reach the prescribed chemical composition, it is difficult for the precipitates to precipitate. It is done. Therefore, in the case of a large cast member, it can be said that the precipitation behavior of Alloy 625 as represented in the TTT diagram of FIG.

  The inventors of the present invention have observed the structure of an alloy 625 large-sized cast member subjected to aging heat treatment under various conditions, particularly focusing on the dendrite core and the dendrite boundary. When heat treatment is performed under specific conditions, the dendrite core has almost no structural change, but a large amount of precipitates are seen at the dendrite boundary, the proof stress is improved, and there is no significant decrease in ductility. I found The precipitate was estimated to be δ phase or Laves phase from past reports. That is, according to the present invention, it is unexpected that the δ phase and Laves phase, which were conventionally considered to be embrittled phases, are appropriately precipitated in advance to improve the yield strength of Alloy 625 and have no significant effect on ductility. Based on the discovery.

The present invention is a large cast member made of Alloy 625, wherein (1) δ phase or Laves phase is precipitated at the dendrite boundary and grain boundary, or (2) MC type, M ₂₃ C ₆ type or M ₆ C-type carbide is precipitated. The large cast member of the present invention may have both the features (1) and (2). If both the δ phase and / or Laves phase and carbide are precipitated, it is considered that a more excellent yield strength improving effect can be obtained. In addition to these, a γ ″ phase may be precipitated.

The main components of the intermetallic compounds constituting the γ ″ phase, δ phase and Laves phase, and MC type, M ₂₃ C ₆ type or M ₆ C type carbide in Alloy 625 are known to those skilled in the art. Two types of intermetallic compounds are known. In particular, it is preferable that three or more types, particularly four or more types, are precipitated, and one or more types of carbides are preferably precipitated.

  The “large cast member” in this specification can be defined by, for example, the member weight or the thickness of the maximum thickness portion. The weight of the member corresponds to 1 ton or more, particularly 5 ton or more, especially 10 ton or more. If it is the maximum thickness part, the thing of 50 mm or more, especially 100 mm or more, especially 200 mm or more corresponds. In the present specification, “large cast member” means a member corresponding to at least one of the above definitions.

  Specific examples of the large cast member in the present invention include those used particularly at high temperatures such as a turbine casing, a turbine valve casing, a nozzle box, and an elbow in a steam turbine. The large cast member of the present invention is suitable as a member of a steam turbine because it does not easily change a large characteristic even when exposed to a high temperature of 680 ° C. or higher, particularly 700 ° C. or higher, particularly 750 ° C. or higher for a long time.

  The large cast member of the present invention can be produced by a method including a step of casting using Alloy 625 and a step of heat-treating the obtained cast member at 700 to 820 ° C. for 8 to 200 hours.

Casting can be performed in a vacuum atmosphere or an inert atmosphere such as argon. After casting, a normal solution treatment is performed, followed by heat treatment (aging heat treatment). The solution treatment is performed, for example, by heating at 1050 to 1250 ° C. for 1 to 20 hours and then cooling with water. By the heat treatment after the solution treatment, δ phase or Laves phase and / or MC type, M ₂₃ C ₆ type or M ₆ C type carbide contributing to the improvement of the resistance are precipitated.

  The heat treatment of the cast member requires a long time for the heat treatment at a temperature lower than 700 ° C. because the precipitation rate is slow, and only a γ ″ phase is precipitated in a short time. On the other hand, at a temperature higher than 820 ° C. Since the speed is high, there is a concern that the ductility may be lowered.Therefore, the heat treatment conditions are 700 to 820 ° C. (more preferably 750 to 800 ° C., more preferably 760 to 800 ° C.) in consideration of material characteristics and manufacturing cost. 8 to 200 hours (more preferably 16 to 120 hours).

  In order to precipitate the intermetallic compound more efficiently, the heat treatment can be performed in two stages. For example, heat treatment is performed at 780 to 820 ° C. in which the δ phase and Laves phase are efficiently precipitated in the first stage to generate nuclei, and then 700 to 750 ° C. in order to increase the δ phase and Laves phase precipitated in the second stage. Heat treatment can be performed. The heat treatment time is preferably 1 to 4 hours for the first stage and 4 to 120 hours for the second stage.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, this invention is not limited to these Examples.

  A large-sized member made of Inconel 625 (IN625) produced by casting was heat-treated at a temperature in the range of 650 to 850 ° C. The heat treatment time was in the range of 8 to 200 hours. After heat treatment, a tensile test was performed at room temperature in accordance with JIS Z 2241 to measure 0.2% yield strength. Moreover, 0.2% yield strength was similarly measured about the thing which did not heat-process as a control | contrast. The results are summarized in Table 1.

  At 650 ° C., the precipitation rate of the precipitates was slow, and a sufficient improvement in strength was not observed. On the other hand, at 850 ° C., although the yield strength was improved, the decrease in ductility measured separately was significant. When the heat treatment temperature was in the range of 700 ° C. to 820 ° C., the desired yield strength improvement effect was obtained, and no particular problem was found in ductility. The effect of improving the yield strength was seen from 8 hours, and when the heat treatment temperature was 820 ° C., the effect of improving the yield strength was seen in 8 hours. In addition, when the heat treatment temperature was in the range of 700 to 800 ° C., particularly when the heat treatment time was 48 hours or more, the effect of improving the yield strength was observed.

Claims (5)

Cr: 20-23% by weight, Mo: 8-10% by weight, Nb + Ta: 3.15-4.15% by weight, Fe: 0-5% by weight, Co: 0-1% by weight, Ti: 0-0. 4% by weight, Al: 0 to 0.4% by weight and a nickel-base alloy having a composition containing inevitable impurities,
(1) δ phase or Laves phase is precipitated at the dendrite boundary and grain boundary, or (2) at least MC type, M ₂₃ C ₆ type or M ₆ C type carbide is precipitated at the grain boundary. A large cast member having any of the characteristics.   The large cast member according to claim 1, which has both the features (1) and (2). Cr: 20-23% by weight, Mo: 8-10% by weight, Nb + Ta: 3.15-4.15% by weight, Fe: 0-5% by weight, Co: 0-1% by weight, Ti: 0-0. 4% by weight, Al: 0 to 0.4% by weight and a step of casting using a nickel-base alloy having a composition containing inevitable impurities, and the obtained cast member is subjected to 700 to 820 after solution treatment. The manufacturing method of the large sized cast member of Claim 1 or 2 including the process heat-processed at 8 degreeC for 8-200 hours.   4. The method according to claim 3, wherein the heat treatment comprises a two-step heat treatment, a heat treatment temperature of the first step is 780 to 820 ° C., and a heat treatment temperature of the second step is 700 to 750 ° C. 5.   The large cast member according to claim 1 or 2, which is a turbine casing, a turbine valve casing, a nozzle box or an elbow member of a steam turbine.

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